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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-25F853BF-BA95-5348-809E-670C04734AB7" xml:lang="en"><title>How
to Choose a Real-time Thread Priority</title><shortdesc>Explains the factors to consider when choosing real time thread
priority.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<ul>
<li id="GUID-5CF494FB-C569-5B38-AB43-23F489C6F41F"><p> <xref href="GUID-25F853BF-BA95-5348-809E-670C04734AB7.dita#GUID-25F853BF-BA95-5348-809E-670C04734AB7/GUID-80550A0E-CB7D-5988-B07D-4A1542501EF2">Real-time categories</xref> </p> </li>
<li id="GUID-A81CB275-5C5B-5155-821A-ED6E803FC322"><p> <xref href="GUID-25F853BF-BA95-5348-809E-670C04734AB7.dita#GUID-25F853BF-BA95-5348-809E-670C04734AB7/GUID-AEE56C03-9346-54AF-9E80-A00F5856663D">Guidelines for writing real-time code</xref> </p> </li>
</ul>
<section id="GUID-80550A0E-CB7D-5988-B07D-4A1542501EF2"><title>Real-time categories</title> <p>Real-time
tasks divide into two categories: </p> <ul>
<li id="GUID-C773539E-F061-59A0-9777-D41175DF2475"><p> Hard real-time </p> </li>
<li id="GUID-8340689D-F9AB-561F-BAA1-C966EB07F352"><p> Soft real-time </p> </li>
</ul> </section>
<section id="GUID-13F43E08-7E46-4A66-BF36-7516399838DC"><title>Hard real-time</title><p>Hard real-time tasks <i>must</i> complete
within a fixed amount of time otherwise they fail. </p> <p>Audio playback
provides a good example of a hard real-time task. During audio playback the
soundcard driver reads data from an audio buffer to transfer to the soundcard.
The audio application uses a thread to re-fill this buffer before it empties,
and failure to do this before the deadline causes an audible break in the
sound. </p> <p>Telephony applications are another example of hard real-time
tasks. </p> <p>Symbian platform gives no real-time scheduling guarantees to
user processes. Instead, system threads such as those running device drivers
are designed to be efficient and well behaved for their priorities. The following
list gives the estimated deadline ranges other threads are likely to meet
reliably for a given priority on a busy system: </p> <table id="GUID-86D3C2BE-C5B4-5926-91C3-712E37C91968">
<tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>
<tbody>
<row>
<entry><p> <b> Deadline range </b>  </p> </entry>
<entry><p> <b> True thread priority value</b>  </p> </entry>
</row>
<row>
<entry><p>&gt; 100ms </p> </entry>
<entry><p>24-26 </p> </entry>
</row>
<row>
<entry><p>&gt; 20ms </p> </entry>
<entry><p>27 </p> </entry>
</row>
<row>
<entry><p>2-20ms </p> </entry>
<entry><p>28-31 </p> </entry>
</row>
</tbody>
</tgroup>
</table> <p>These figures rely on real-time threads completing their work
in less time than the deadline itself, otherwise other threads may fail to
meet their deadlines. These deadline ranges appear conservative because they
are based on what is a achievable on a busy system. Use of lower <i>true</i> priorities
than those in the above table are recommended whenever the implications of
failing to meet a deadline are not very serious and the incidence of failure
is acceptably low in practical testing. </p> </section>
<section id="GUID-AB70E058-6910-4ECB-A84C-1639D56381C9"><title>Soft real-time</title><p>Soft real-time tasks <i>try</i> to
complete within a fixed amount of time. In contrast to hard real-time, exceeding
this time is not considered a failure, although it may affect the user experience
in some way. </p> <p>Soft real-time tasks generally have a quality of service
target expressed as a percentage that is less than 100% (where 100% represents
hard real-time). This value is the percentage of tasks that must complete
within the deadline to satisfy the expected service levels. Usually the programmer
implements a recovery strategy to cope with a thread missing the deadline. </p> <p>Examples
of soft real-time tasks include the responsiveness of the system to user events
and communications protocols. In the case of responsiveness to user events,
an arbitrary deadline is aimed for that is ‘close enough to instantaneous’;
in the case of communication protocols, failure to process data in time causes
it to be retransmitted – possibly at extra cost to the user – but is otherwise
not a catastrophic failure. </p> </section>
<section id="GUID-AEE56C03-9346-54AF-9E80-A00F5856663D"><title>Guidelines
for writing real-time code</title> <p>When developing real-time code follow
these rules: </p> <ul>
<li id="GUID-23007425-8BA8-5AD3-B7CE-7EEB56FDA701"><p>Do the minimum amount
of work required to meet your deadline at the required priority. </p> </li>
<li id="GUID-DDB7AF0E-9245-5511-A6C0-FCA5B6928E9E"><p>For hard real-time tasks,
use the lowest priority you can to achieve your goals within the task deadline. </p> </li>
<li id="GUID-2732E0E6-5CAA-502A-882D-195F497019BF"><p>Avoid using lower priority,
non real-time services, for example file I/O in high priority threads. This
sacrifices the benefit of using a high priority real-time thread. </p> </li>
<li id="GUID-6EC4A4CB-4EBE-5F5B-BC42-C6A926181FAC"><p>For soft real-time tasks
choose a priority above which no significant performance differences are observed
once the required service level is met. </p> </li>
<li id="GUID-329AD22C-052C-5E81-A79B-8747E2D4586E"><p>For typical applications
that respond to user interaction, soft real-time priorities start at values
based on the <xref href="GUID-1054385C-7168-31ED-89D9-4C33E28836A0.dita"><apiname>EPriorityAbsoluteForegroundNormal</apiname></xref> process
priority, but are more often based on the <xref href="GUID-8995D880-69B7-3C31-891B-7A2C57E372FE.dita"><apiname>EPriorityHigh</apiname></xref> process
priority. </p> </li>
<li id="GUID-EC60209A-BFCF-58E2-BD80-8A1DA754CA96"><p>For tasks that will
use all available CPU by the sheer frequency that the thread is scheduled
(for example, using a very high bandwidth communication channel, or achieving
high/maximum frame rates in an action game), lower priorities should be used;
this includes priorities based on the <xref href="GUID-1054385C-7168-31ED-89D9-4C33E28836A0.dita"><apiname>EPriorityAbsoluteForegroundNormal</apiname></xref> process
priority. </p> </li>
<li id="GUID-8C832884-41EF-5F80-8EAA-347A320D9015"><p>Escalating priorities
for soft real-time tasks into the levels normally used for hard real-time
tasks is dangerous. </p> </li>
</ul> </section>
</conbody></concept>